1. Field of the Invention
This invention relates to an achromatic tuner for a birefringent optical filter, including an achromatic birefringent retardation plate therefor. Accordingly, it is a general object of this invention to provide new and improved tuners and filters therefor.
2. General Background
Optical frequency or wavelength selective filters with exacting bandpass characteristics have been used in the past for the isolation of spectral lines. As the optical spectrum is used to an increasing extent for communications channels, such filters are being adapted for the separation of optical signals from a single transmission medium such as optical fiber. Birefringent optical filters are capable of such exacting separation of signals having closely spaced wavelengths, and their designs have progressed to allow the realization of any desired periodic transmission function as well as the simple sinusoidal response obtainable with a single birefringent crystal element located between polarizers. The theory and art of such birefringent optical filters have been reviewed in some detail by A. M. Title and W. J. Rosenberg in their article, "Tunable Birefringent Filters", Optical Engineering, 20(6), pp. 815-823, (1981).
Of particular interest is the Solc-type filter configuration in which a lossless sequence of birefringent crystal elements is located between input and output polarizers. The number and orientation angles of these elements may be chosen to synthesize any periodic filter transmission versus optical frequency characteristic, and by replacing the polarizers with polarizing beam splitters, there is obtained an optical multiplexer/demultiplexer device with this transfer function and its complement as its operating characteristic.
My U.S. patent, "Methods of and Apparatus for Tuning a Birefringent Optical Filter", U.S. Pat. No. 4,678,287, issued July 7, 1987, assigned to the common assignee of this application, describes several configurations of quarter-wave plates that, when used in conjunction with two birefringent crystal elements of such a lossless Solc-type birefringent filter, permit the simultaneous and equal tuning of these two elements by rotating a single contiguous group of optical components. When all birefringent crystal elements of the filter are so tuned in synchronism to effectively add to or subtract from the birefringence of each component element equally, the periodic transmission function becomes shifted without distortion in the optical frequency domain.
Quarter-wave plates are the essential components of the two plate tuners used to tune the birefringent crystal elements. A relative rotation between the plates gives both the desired tuning effect to the associated birefringent crystal element and an undesired rotational reorientation between opposite ends of the optical system where the input and output polarizers or polarizing beam splitters are located. The latter can be eliminated by configuring a pair of two plate tuners in a complementary manner so that equal and opposite relative rotations between their component quarter-wave plates give equal additions to the birefringence of their associated crystal element. No net rotational reorientation between the optical system input and output remains when such complementary pairs of two plate tuners are used. This is particularly desirable when polarizing beam splitters are being used to implement multiplexer devices.
Ideal quarter-wave plates propagate unchanged two components of plane polarized light, one with its electric field along the fast axis of the plate, and the other with its electric field along the slow axis of the plate. Upon passing through the plate, the two waves undergo a differential phase shift of ninety degrees such that the light polarized along the fast axis is phase advanced relative to the light polarized along the slow axis. Practical quarter-wave plates can consist of a uniaxially stressed thin film of certain plastics such as polyvinyl alcohol, or preferably can be optically polished thin parallel plates or disks cut from a birefringent crystal such as quartz. Their thickness is chosen to give the desired ninety degree phase shift between the ordinary and extra-ordinary polarization components.
Optical filtering devices, designed to operate over a wavelength range about a nominal center wavelength, include components such as polarizers, polarizing beam splitters, and especially quarter-wave plates used for tuning that should perform well over the wavelength range. For example, a quartz wave plate 38.23 microns thick and cut such that the optic axis is perpendicular to the plate normal, has a nominal phase retardation of ninety degrees at 1.315 microns. When it is used in tuning a birefringent filter operating in the 1.28 to 1.35 micron range, its retardation at these limiting wavelengths is of interest because it affects both the performance of the tuner and that of the filter. The retardation, in degrees, for a uniaxial positive crystal such as quartz is given by the formula EQU .delta.=360t(n.sub.e -n.sub.o)/.lambda..sub.o
where t is the plate thickness in microns, .lambda..sub.o is the nominal center wavelength in microns, and n.sub.o and n.sub.e are the ordinary and extra-ordinary refractive indices. At 1.28 and 1.35 microns, the retardation is 92.68 and 87.46 degrees, respectively. This deviation from 90 degrees is small in the wavelength range of interest, but can result in a noticeable degradation in the birefringent filter performance when several such quarter-wave plates are used in a multielement filter design.
A quarter-wave plate, as heretofore described, is known as a first order plate. Its fabrication from a quartz crystal disk is difficult due to its thinness and fragility. Other crystals with a smaller birefringence may be preferable, but none with the optical quality of quartz is presently readily available. Quartz retardation plates are produced according to the prior art in two ways. In one, the disk is made some odd multiple n of the above thickness to yield a high order plate of n quarter waves. At its design center wavelength of 1.315 microns, for example, it has a retardation of 90 n degrees, which is equivalent to a quarter-wave plate when n=1,5,9,13, etc. and to a reversed (fast and slow axes reversed) quarter-wave plate when n=3,7,11, etc. As the wavelength deviates from the design center, the plate retardation changes from quarter-wave at a rate n times as fast as that of a first order quarter-wave plate. Disadvantageously, this type of plate is unsatisfactory for constructing tuners for birefringent filters. In the other method of the prior art, a composite plate is constructed of two parallel quartz disks in which the retardation values differ by ninety degrees. Each is oriented with its optic axis perpendicular to the common plate normal and to the optic axis of the other disk so that the birefringence effects of each partially cancel one another. The net retardation is ninety degrees, and, because the retardation changes with wavelength at the same rate as a first order plate, the composite is termed a pseudo first order quarter-wave plate.
The combination of simple wave plates to form achromatic wave plates, usually with the goal of achieving quarter- or half-wave plate qualities over the visible spectrum, has been described in the prior art. These so-called combination wave plates were described by G. Destriau and J. Prouteau (J. Phys. Radium, Ser. 8, Vol. 10, p. 53, 1949). Further work was done by S. Pancharatnam (Proc. Indian Acad. Sci., A41, 137, 1955). Their development for the tuning of birefringent filters was continued by A. M. Title (Applied Optics, 14(1), 229, Jan. 1975). The achromatic birefringence obtained is often accompanied by a nonzero optical rotatory effect which is expected on the basis of a theorem proven by R. C. Jones who showed that for a given wavelength any combination of retardation plates is equivalent to, at most, a linear retardation and a rotation (J. Optical Soc. Amer., 31, p. 493, 1941). Usually, three or more plates should be combined if the resulting achromatic combination is to be free of this equivalent rotation component.
The requisite first order plates from which such composite achromatic quarter-wave plates could be made would themselves need to be composite pseudo first order plates if they are to be made from a birefringent crystalline material such as quartz.